1. Field of the Invention
The present invention relates to surface acoustic wave devices and communication apparatuses, and more particularly, to a surface acoustic wave device for use in, for example, a communication apparatus, such as a portable telephone set, and a communication apparatus including the surface acoustic wave device.
2. Description of the Related Art
In recent years, the demand for providing surface acoustic wave filters used for an RF stage of portable telephone sets with a balanced-unbalanced conversion function, the so-called balun function, has been increasing. In particular, longitudinally coupled resonator type surface acoustic wave filters that are easily adaptable to balanced-unbalanced signal conversion have become mainstream as band pass filters for an RF stage of portable telephone sets. The longitudinally coupled resonator type surface acoustic wave filter with such a balanced-unbalanced conversion function is connected to a mixer IC (hereinafter, referred to as a balanced mixer IC) having balanced or differential input and output. The use of such a balanced mixer IC achieves a reduction in susceptibility to noise and realizes output stabilization. Thus, the characteristics of a portable telephone set are also improved. Therefore, recently, the balanced mixer IC has been used frequently.
Although a surface acoustic wave filter used for an RF stage normally has an impedance of 50 Ω, the balanced mixer IC mostly has a high impedance, such as approximately 100 to 200 Ω. Since an impedance of 200 Ω has been mainstream, a characteristic exhibiting an approximately four times difference between an input impedance and an output impedance has been required for a longitudinally coupled resonator type surface acoustic wave filter. Also, characteristics exhibiting low loss and high attenuation are often required for a band pass filter for an RF stage.
In order to make an approximately four times difference between an input impedance and an output impedance and to achieve high attenuation, a method disclosed in Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-117123 is widely used. A surface acoustic wave device (Conventional Example 1) disclosed in Patent Document 1 includes two two-stage connection surface acoustic wave filters 105 and 106, as shown in FIG. 35. In the two-stage connection surface acoustic wave filter 105, surface acoustic wave elements 101 and 103 are cascaded to each other. In the two-stage connection surface acoustic wave filter 106, surface acoustic wave elements 102 and 104 are cascaded to each other. The two-stage connection surface acoustic wave filters 105 and 106 have a balanced-unbalanced conversion function by changing the phase of the surface acoustic wave element 104 by 180 degrees. Also, one terminal of the two-stage connection surface acoustic wave filter 105 and one terminal of the two-stage connection surface acoustic wave filter 106 are electrically connected in parallel to an unbalanced terminal 111. The other terminal of the two-stage connection surface acoustic wave filter 105 and the other terminal of the two-stage connection surface acoustic wave filter 106 are connected in series to balanced terminals 112 and 113, respectively.
Not only is the balanced mixer IC required to have an impedance of 200 Ω, the range of a required impedance is being extended to 100 Ω or 150 Ω. In accordance with such a requirement, there is a necessity to design a longitudinally coupled resonator type surface acoustic wave filter with a balanced-unbalanced conversion function such that the input to output impedance ratio is 1:2 or 1:3. For example, in a surface acoustic wave device (Conventional Example 2) according to Patent Document 2: Japanese Patent No. 3239064 (Japanese Unexamined Patent Application Publication No. 9-321574), the impedance ratio of input terminal to output terminal can be set to a desired value by making a cross width W1 of a surface acoustic wave element 201 including interdigital transducers (hereinafter, referred to as IDTs) 203 to 205 different from a cross width W2 of a surface acoustic wave element 202 including IDTs 206 to 208, as shown in FIG. 36. In the surface acoustic wave elements 201 and 202, the IDTs 203 and 206 are cascaded to each other, and the IDTs 205 and 208 are cascaded to each other. Also, the surface acoustic wave elements 201 and 202 are connected to an input terminal 211 and an output terminal 212, respectively.
However, the arrangement of the surface acoustic wave device shown in FIG. 36 has a problem in that the voltage standing wave ratio (VSWR) is deteriorated. One of the causes of this problem is that making the cross width W1 of the surface acoustic wave element 201 different from the cross width W2 of the surface acoustic wave element 202 causes the difference in the impedances of the cascaded IDTs 203 and 206 and the difference in the impedances of the cascaded IDTs 205 and 208 and that the impedance differences cause mismatching in a joined surface of the surface acoustic wave elements 201 and 202. Thus, ideally, it is desirable that only the impedance of the IDT 204, which is connected to the input terminal 211 of the surface acoustic wave device, and the impedance of the IDT 207, which is connected to the output terminal 212 of the surface acoustic wave device, shown in FIG. 36, be adjusted. However, in a case where an impedance adjustment is performed by changing the cross widths only of the IDTs 204 and 207, a leakage of a surface acoustic wave is caused in a transmission path for surface acoustic waves, thus deteriorating the characteristics. Accordingly, it is difficult to set the impedance ratio of the input and output terminals 211 and 212 to a desired value without deteriorating the characteristics by making the cross width W1 of the surface acoustic wave element 201 different from the cross width W2 of the surface acoustic wave element 202.
An adjustment of the impedance ratio of unbalanced terminal to balanced terminal performed by applying the arrangement of the surface acoustic wave device shown in FIG. 36 to the surface acoustic wave device shown in FIG. 35 will now be studied.
FIGS. 37 to 39 show characteristics of the longitudinally coupled resonator type surface acoustic wave filters shown in FIG. 35 having a balanced-unbalanced conversion function with an unbalanced terminal impedance of 50 Ω and a balanced terminal impedance of 200 Ω. These characteristics are provided by the surface acoustic wave device shown in FIG. 35 designed as a personal communication system (PCS) receiving filter and the frequency range necessary for the passband is between 1,930 MHz and 1,990 MHz. Here, the cross width W1 of the surface acoustic wave elements of the longitudinally coupled resonator type surface acoustic wave filters connected on the unbalanced terminal side is set to be equal to the cross width W2 of the surface acoustic wave elements of the longitudinally coupled resonator type surface acoustic wave filters connected on the balanced terminal side. FIG. 37, FIG. 38, and FIG. 39 show the transmission characteristics, the impedance characteristics, and the VSWR, respectively, near the pass band. In FIGS. 38 and 39, reference numeral S11 represents an input side and reference numeral S22 represents an output side. As is clear from FIG. 38, the impedance ratio of unbalanced terminal to balanced terminal is approximately 1:4. Also, as is clear from FIG. 39, excellent characteristics exhibiting a VSWR of approximately 1.7 in the pass band can be achieved.
FIGS. 40 to 42 show the characteristics of the surface acoustic wave device shown in FIG. 35 when the cross width W1 of the surface acoustic wave elements of the longitudinally coupled resonator type surface acoustic wave filters connected on the unbalanced terminal side is set to be different from the cross width W2 of the surface acoustic wave elements of the longitudinally coupled resonator type surface acoustic wave filters connected on the balanced terminal side, as disclosed in Patent Document 2, in order to set the impedance ratio of unbalanced terminal to balanced terminal to 1:3. In order to set the impedance ratio of unbalanced terminal to balanced terminal to 1:4 and 1:3, the cross width W1 must be smaller than the cross width W2. Here, the cross widths W1 and W2 are set so as to satisfy the condition W2/W1≈1.57. FIG. 40, FIG. 41, and FIG. 42 show the transmission characteristics, the impedance characteristics, and the VSWR, respectively, near the pass band. In FIGS. 41 and 42, reference numeral S11 represents an input side and reference numeral S22 represents an output side. As is clear from FIG. 42, the VSWR is deteriorated to approximately 2.3 in the pass band since matching cannot be achieved with a desired impedance ratio of unbalanced terminal to balanced terminal (50 Ω:100 Ω).
Accordingly, it is also difficult for the surface acoustic wave device with the arrangement of a combination of the arrangement shown in FIG. 35 (Conventional Example 1) and the arrangement shown in FIG. 36 (Conventional Example 2) to set the impedance ratio of unbalanced terminal to balanced terminal to 1:2 or 1:3. One of the causes of this problem is that, similarly to the problem caused in the surface acoustic wave device shown in FIG. 36, making the cross widths of the surface acoustic wave elements different from each other causes the difference in the impedances of the cascaded IDTs and that the impedance difference causes impedance mismatching in a joined surface of the two surface acoustic wave elements. Thus, similarly to the surface acoustic wave device with the arrangement shown in FIG. 36, it is also difficult for the surface acoustic wave device with the arrangement of a combination of the arrangement shown in FIG. 35 and the arrangement shown in FIG. 36 to set the impedance ratio of input terminal and output terminal to a desired value without deteriorating the characteristics by making the cross widths different from each other.
Thus, a method for achieving matching with an approximately two or three times difference between the impedances of an unbalanced terminal and a balanced terminal by adding a matching element outside a surface acoustic wave device, such as, adding an inductance element in parallel and a capacitance element in series on a balanced terminal side, apart from the design of the surface acoustic wave device, has been used. However, this method has a problem in that the number of component elements is increased due to addition of an external element and the miniaturization is thus prevented. Thus, this is inappropriate for current requirements.